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multiple VMs are concurrently running on the same physical host,
each VM mayexhibit a varying and unstable performance (Speed of
Execution, and not results), which highly depends on the workload
imposed on the system by other VMs, unless proper techniques are
used for temporal isolation among virtual machines.
Multiple VMs each running its own operating system (called guest
operating system) are frequently used in server consolidation,
where different services that used to run on individual machines to
avoid interference are instead run in separate VMs on the same
physical machine.
The desire to run multiple operating systems was the original
motivation for virtual machines, as it allowed time-sharing a
single computer between several single-tasking Operation Systems.
In some respects, a system virtual machine can beconsidered a
generalization of the concept of virtual memory that historically
preceded it. IBM's CP/CMS, the first systems to allow full
virtualization, implemented time sharing by providing each user
with a single-user operating system, the CMS. Unlike virtual
memory, a system virtual machine allowed the user to useprivileged
instructions in his code. This approach had certain advantages, for
instance it allowed users to add input/output devices not allowed
by the standardsystem.[4]
As technology evolves virtual memory, in respect to
virtualization, will utilizethe technologies of memory
overcommitment to manage the memory sharing betweenmultiple virtual
machines on one physical computer. On a related note, sometimes
it is possible to share those memory pages that have identical
contents among multiple virtual machines running on the same
physical machine, mapping them to the same physical page, by a
technique known as Kernel SamePage Merging. This isparticularly
useful for read-only pages, such as those ones containing code
segments, especially in the case of multiple virtual machines
running the same or similar software, such as the Operating System,
software libraries, web server, middleware components, etc..
The guest OSes do not have to be compliant with the hardware
making it possibleto run different OSes on the same computer (e.g.,
Microsoft Windows and Linux, or older versions of an OS to support
software that has not yet been ported to the latest version). The
use of virtual machines to support different guest OSes is becoming
popular in embedded systems; a typical use is to support a
real-time
operating system at the same time as a high-level OS such as
Linux or Windows.
Another use is to sandbox an OS that is not trusted, possibly
because it is a system under development. Virtual machines have
other advantages for OS development, including better debugging
access and faster reboots.[5]Process virtual machinesSee also:
Application virtualization, Run-time system, and Comparison of
application virtual machines
A process VM, sometimes called an application virtual machine,
runs as a normalapplication inside a host OS and supports a single
process. It is created when that process is started and destroyed
when it exits. Its purpose is to provide aplatform-independent
programming environment that abstracts away details of the
underlying hardware or operating system, and allows a program to
execute in thesame way on any platform.
A process VM provides a high-level abstraction that of a
high-level programminglanguage (compared to the low-level ISA
abstraction of the system VM). Process VMs are implemented using an
interpreter; performance comparable to compiled programming
languages is achieved by the use of just-in-time compilation.
This type of VM has become popular with the Java programming
language, which isimplemented using the Java virtual machine. Other
examples include the Parrot vi
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rtual machine, which serves as an abstraction layer for several
interpreted languages, and the .NET Framework, which runs on a VM
called the Common Language Runtime.
A special case of process VMs are systems that abstract over the
communication mechanisms of a (potentially heterogeneous) computer
cluster. Such a VM does notconsist of a single process, but one
process per physical machine in the cluster. They are designed to
ease the task of programming parallel applications by letting the
programmer focus on algorithms rather than the communication
mechanismsprovided by the interconnect and the OS. They do not hide
the fact that communication takes place, and as such do not attempt
to present the cluster as a single parallel machine.
Unlike other process VMs, these systems do not provide a
specific programming language, but are embedded in an existing
language; typically such a system provides bindings for several
languages (e.g., C and FORTRAN). Examples are PVM (Parallel Virtual
Machine) and MPI (Message Passing Interface). They are not
strictlyvirtual machines, as the applications running on top still
have access to all OSservices, and are therefore not confined to
the system model provided by the "VM"
